I've tried the Array Column question and the problem is the other way around i.e. an attribute can be selected as Most Important and 2nd Most important.
I'm happy to do it with some Javascript if someone can show me how.

Add this script to the source of the question text or help. Replace "QQ" with your question ID.

The script puts a listener on the answer cells. When one is clicked, if there are any others with the same class (same column) that have a checked radio, an alert is popped up and the radio is unchecked.